Each satellite that orbits the Earth can have a distinct ephemeris. The ephemeris can include various types of information, such as the position or orbit of the satellite on a number of dates and times in a regular sequence. Some satellites can utilize knowledge of their orbit information for various applications. For example, a satellite's orbit information can be used to enhance the utility of altimeter and imaging system applications. In other cases, the operators of a satellite need to know its current position and orbital parameters to adjust the pointing of antennas or other sub-systems, or to adjust the orbit of the satellite to a more favorable location.
A significant number of satellites have orbits at or near zero degrees of inclination at an altitude that provides a 24 hour orbital period. These so called Geosynchronous Satellites (GS) appear to move little or none to observers on the ground. Orbit determination (OD) of satellites is typically computed by receiving 4 or more GPS signals.
Operators may uplink a signal from a ground station to the GS satellite and receive back a transponded, downlinked version of that signal. The time of flight of the uplink and downlink provide the operator range from the ground station to the GS satellite, but little other information on its orbital characteristics. Many observations, over an extended period of time (e.g. hours) must be made to estimate the orbit of the GS using the transponded signal.
Satellites that are not GS satellites may also receive an uplink from a ground station and transpond that signal back to the ground station. The signal received back at the ground will have appreciable Doppler shift compared to the original transmission. Multiple samples of the Doppler Shift can be used to estimate the orbit of the satellite. Unfortunately the Doppler Shift measured on the transponded signal from a GS will generally be too small to provide useful orbital information.
Satellites can communicate with ground stations fromr time to time. Some satellites, especially GS satellites, are in constant radio communication with one or more ground stations. Many GS satellites can broadcast additional radio signals to convey communications or to relay collected data to the ground.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.